1. Field of the invention
The present invention relates to microwave resonators, and particularly to microwave resonators which are passive devices for handling electromagnetic waves having a very short wavelength such as microwaves and millimetric waves, and which have conductor layers, a portion of which is formed of an oxide superconductor material.
2. Description of related art
Electromagnetic waves called "microwaves" or "millimetric waves" having a wavelength in a range of a few centimeters to a few millimeters can be said from a viewpoint of physics to be merely a part of an electromagnetic wave spectrum, but have been considered from a viewpoint of electric engineering to be a special independent field of electromagnetic waves, since special and unique methods and devices have been developed for handling these electromagnetic waves.
Microwaves and millimetric waves are characterized by a straight going property of radio waves, reflection by a conduction plate, diffraction due to obstacles, interference between radio waves, optical behavior when passing through a boundary between different mediums, and other physical phenomena. In addition, the effect of some physical phenomena which are too small to appear in a low frequency electromagnetic wave or in light will remarkably appear in the microwaves and millimetric waves. Thus used an isolator and a circulator utilizing a gyro magnetic effect of a ferrite, and medical instruments such as plasma diagnosis instrument utilizing interference between a gas plasma and a microwave are row used. Furthermore, since the frequency of the microwaves and millimetric waves is extremely high, the microwaves and millimetric waves are used as a signal transmission medium of a high speed and a high density.
In the case of propagating an electromagnetic wave in frequency bands which are called the microwave and the millimetric wave, a twin-lead type feeder used in a relative low frequency band has an extremely large transmission loss. In addition, if an inter-conductor distance approaches a wavelength, a slight bend of the transmission line and a slight mismatch in the connection portion will cause reflection and radiation, and is easily influenced from adjacent objects. Thus, a tubular waveguide having a sectional size comparable to the wavelength has been used. The waveguide and a circuit comprising of the waveguide constitute a three-dimensional circuit, which is larger than components used in ordinary electric and electronic circuits. Therefore, application of the microwave circuit has been limited to special fields.
However, miniaturized devices composed of semiconductor have been developed as an active element operating in a microwave band. In addition, with advancement of integrated circuit technology, a so-called microstrip line having an extremely small inter-conductor distance has been used.
In 1986, Bednorz and Muller discovered (La, Ba).sub.2 CuO.sub.4 showing a superconduction state at a temperature of 30 K. In 1987, Chu discovered YBa.sub.2 Cu.sub.3 O.sub.y having a superconduction critical temperature on the order of 90 K., and in 1988, Maeda discovered a so-call bismuth (Bi) type compound oxide superconductor material having a superconduction critical temperature exceeding 100 K. These compound oxide superconductor materials can obtain a superconduction condition with cooling using an inexpensive liquid nitrogen. As a result, possibility of actual application of the superconduction technology has become discussed and studied.
Phenomenon inherent to the superconduction can be advantageously utilized in various applications, and microwave components are no exceptions. In general, the microstrip line has an attenuation coefficient that is attributable to a resistance component of the conductor. This attenuation coefficient attributable to the resistance component increases in proportion to a root of a frequency. On the other hand, the dielectric loss increases in proportion to increase of the frequency. However, the loss of the microstrip line particularly in the range of microwaves and millimetric waves is almost attributable to the resistance of the conductor, since the dielectric materials have been improved. Therefore, if the resistance of the conductor in the strip line can be reduced, it is possible to greatly elevate the performance of the microstrip line.
As is well known, the microstrip line can be used as a simple signal transmission line. However, if a suitable patterning is applied, the microstrip line can be used as an inductor, a filter, a resonator, a directional coupler, and other passive microwave circuit elements that can be used in a hybrid circuit.
EP-A2-0 357 507 published on Mar. 7, 1990 discloses microwave waveguides using an oxide superconductor material. However, a practical microwave resonator utilizing an excellent property of the oxide superconductor material has not yet been proposed.